Pigments are widely used in many industries to impart basically a pleasing, esthetic color and appearance to various media in which they are mixed, such as paints, varnishes, organic resins, glazes, ceramic ware such as porcelain enamels, and the like. As a general rule, inorganic pigments have a greater scope of application as compared to organic pigments. Because of the more severe and drastic conditions of use in glazes and ceramic applications, particularly with reference to temperature of preparation, inorganic pigments must be used for such applications. However, inorganic pigments may also be added to organic media, such as organic resinous polymeric materials, especially when such materials are used at relatively high temperature such as in extruding or molding.
It is known to us a spinel as an inorganic pigment. Crystallographically, spinels are face-centered cubes or oxygen ions into which metal ions can be located in either tetrahedral or octahedral coordination spheres. Spinels comprise metal oxides in recognized groupings, usually expressed by formulas. A normal spinel crystal structure, for example, may be expressed as AB.sub.2 O.sub.4 in which A may represent one ion or atom of a divalent metal and B represents two ions of a trivalent metal. It will be noted that the sum of the metal positive valences, eight, equals the total negative valence of the oxygen to maintain the spinel in electrical neutrality.
Prior work for developing spinels as inorganic pigments has been confined to the substitution of metal ions of similar charge in a host crystal system. There are two ways in which the substituted metal ion can fit into the spinel crystal structure. In one, the substituted ion fits into an occupied or unoccupied tetrahedral site within the crystal. In another, the substituted ion fits an occupied or unoccupied octadedral position in the host structure. While the resulting spinel crystal lattice structures in both instances have been referred to as solid solutions, they actually consist of only an original, single spinel crystal structure with one or more metal ions substituted in crystal sites normally occupied by other metal ions.
Thus, U.S. Pat. No. 2,139,686 to Lederle discloses forming crystals of the spinel type from meta and ortho zinc titante compositions in which a part of the zinc can be isomorphorically replaced by magnesium. Similarly, although U.S. Pat. 3,424,551 to Owen speaks of a pigment having a characteristic spinel structure and describes these spinels as solid solutions, this patent shows that only a single spinel crystal structure is intended by disclosing only one spinel formula, and by stating that the spinels pertaining to the invention of that patent may be represented by the formula A.sup.x B.sup.y.sub.2 O.sub.4, in which the ionic charges of the A and B cations are such that the sum of these charges (1x + 2y) equals eight; (column 1, lines 36 to 39). In like manner, U.S. Pat. No. 3,876,441 to Broll et al discloses a pigment having a spinel structure but limited to an alkali metal tinanate in which cobalt, zinc and cadmium are substituted in varying amounts, the ion substitutions of the three metals totaling one per molecule of the spinel, as expressed by a single formula, and in which tin may also be substituted for up to 10 mole percent of the titanium.
In short, all of the three identified patents relate only to a spinel crystal system in which metal ion substitutions may be made, that is, one metal ion for another.